1. Field of the Invention
The present invention relates generally to a method and apparatus for displaying a three-dimensional image, and more particularly, to a method and apparatus for displaying a three-dimensional image as if the inside of an object was observed under an endoscope.
2. Description of the Related Art
There are a variety of medical images such as an X-ray Computerized Tomography (CT) image, a Magnetic Resonance Imaging (MRI) image, and an ultrasonic tomographic image. In order to obtain a three-dimensional image, the X-ray CT images are stacked slice by slice. Because the MRI image can be measured three-dimensionally, the three-dimensional original image can be obtained by arranging the MRI images.
In a central projection method, a three-dimensional original image interposed between a viewpoint and a projection plane is projected onto the projection plane as if the inside of an object was observed from the viewpoint. In the central projection method, the viewpoint is neither a plane nor a line but a point. The three-dimensional original image interposed between the viewpoint and the projection plane is projected onto the projection plane, with the viewpoint being the center. For example, in order to obtain the three-dimensional original image showing the inside of an intestine, a viewpoint is placed at the inside of the intestine. Then the line of sight is turned from the viewpoint toward the inside of the intestine, so that the inside of the intestine can be observed. In this case, the image is displayed as if the inside of the intestines was observed under an endoscope. The applicant of the present application has already disclosed the central projection method in Japanese Patent Provisional Publication Nos. 7-210704, 7-296184, and 8-16813.
In the central projection method according to the above-mentioned publications, when the viewpoint is placed in an organ, the image shows only one part of the organ. Thus, this central projection method has such a disadvantage that which part of the organ is displayed cannot be easily recognized.